Conventional wisdom says that the upcoming election for the Board of Supervisors will be a major snooze.

Since ranked-choice voting premiered in 2004, no incumbent has ever lost. Five of the six are running for a second term (Supervisor Sean Elsbernd, who represents District Seven, is termed out).

But don't mark the incumbents down as winners just yet.

There are some interesting factors in play, from the potential rise of a Chinese voting bloc to well-positioned challengers. It is just a reminder that there's no such thing as a dull election in San Francisco.

For starters, Supervisors David Campos and David Chiu are virtually unchallenged. Chiu is still running hard, but speculation is that he just wants to make it clear to anyone considering challenging him that it probably won't be worth it.

After that, all bets are off.

The real surprise is that John Avalos may face a stern test for a second term in District 11. Avalos was anointed the progressive standard-bearer after his second-place finish in the mayor's race. He's no far-left demagogue and has labor credentials. Personable and smart, he's a Latino candidate in a district that includes the Outer Mission.

But the changing demographics tell a different story. Fifty-one percent of the district's residents are now Asian, which was dramatically pointed out when Avalos lost the district lopsidedly to Ed Lee in the mayoral race. Political number-crunchers say if you factor in Leland Yee and Chiu, two other mayoral candidates, Avalos was outpolled 3 to 1 by Chinese candidates.

Leon Chow, Avalos' challenger, not only will tap the Asian vote but also can claim some of the same strengths as Avalos. He counts on union support, but Chow, a longtime resident of the district, is a veteran labor organizer. While Avalos will probably champion his "local hire" initiative, that law was not universally popular with unions, including the building trades, which bitterly opposed it.

Avalos may have a real fight on his hands, particularly if it boils down to a head-to-head battle. Ranked-choice gurus say incumbents have been successful because they've avoided the old system, where the general election produced two challengers for a runoff.

In 2008 in District One, for example, three top contenders polled the majority of the votes, with progressive Eric Mar winning. But Mar had the advantage of running against two moderates, Sue Lee and Alicia Wang, who split their votes and opened the door for a six-point Mar win.

At least for now, Mar is facing a single moderate opponent in David Lee. Lee has all the advantages of Mar - longtime resident of the Richmond, community organizer and Asian background - and never proposed a ban on Happy Meals. That's a reminder that Mar, a likable guy, is battling a perception that he's a little goofy. A recent suggestion that the board members join him in a hot tub didn't help.

The word is Mar's people are trying to recruit someone else to run to split the vote. That's quite a change from the old days when political factions tried to clear the ticket for their candidate.

Finally there is Christina Olague, who answers the question: When is an incumbent not an incumbent? Olague was appointed in January by the mayor to replace Ross Mirkarimi, who went on to become both sheriff and the star of the city's most-popular political reality show.

Olague is a progressive, but she also supported the campaign to draft Lee to run for mayor. Her advantage is that it is shaping up to be a crowded ticket, which traditionally wins elections for incumbents.

As political consultant Eric Jaye says, all the talk of upsets is interesting, but he's skeptical.

"Just remember that under ranked choice the incumbent has enjoyed a tremendous advantage," he says.